They are stored in the TitaniumBackUp folder on the SD card.If you do a new backup, it should update what's already there. You shouldn't need to delete anything...At least that's how I understand it to work..

EDIT: If you have the free version that's how it works. If your using the paid app, you have the option of creating multiple backups of the same app.

Hi:
I don't believe you actually need to look up the file, as Titanium Backup never asks you which file you want to restore from.
If you no longer use the apps and you have deleted them, go into Backup/Restore from Titanium Backup, hit batch and stroll all the way down. There is a command there that says "Delete backups for un-installed apps".
Run that, and your backup file will be updated by TB.
You will see the difference in the number of apps you can now restore.